Multistage nanovectors: from concept to novel imaging contrast agents and therapeutics

Over the last few decades a great variety of nanotechnology based platforms have been synthesized and fabricated to improve the delivery of active compounds to a disease site. Nanoparticles currently used in the clinic, and the majority of nanotherapeutics/nanodiagnostics under investigation, accommodate single- or multiple- functionalities on the same entity. Because many heterogeneous biological barriers can prevent therapeutic and imaging agents from reaching their intended targets in sufficient concentrations, there is an emerging requirement to develop a multimodular nanoassembly, in which different components with individual specific functions act in a synergistic manner. The multistage nanovectors (MSVs) were introduced in 2008 as the first system of this type. It comprises several nanocomponents or "stages", each of which is designed to negotiate one or more biological barriers. Stage 1 mesoporous silicon particles (S1MPs) were rationally designed and fabricated in a nonspherical geometry to enable superior blood margination and to increase cell surface adhesion. The main task of S1MPs is to efficiently transport nanoparticles that are loaded into their porous structure and to protect them during transport from the administration site to the disease lesion. Semiconductor fabrication techniques including photolithography and electrochemical etching allow for the exquisite control and precise reproducibility of S1MP physical characteristics such as geometry and porosity. Furthermore, S1MPs can be chemically modified with negatively/positively charged groups, PEG and other polymers, fluorescent probes, contrast agents, and biologically active targeting moieties including antibodies, peptides, aptamers, and phage. The payload nanoparticles, termed stage 2 nanoparticles (S2NPs), can be any currently available nanoparticles such as liposomes, micelles, inorganic/metallic nanoparticles, dendrimers, and carbon structures, within the approximate size range of 5-100 nm in diameter. Depending upon the physicochemical features of the S1MP (geometry, porosity, and surface modifications), a variety of S2NPs or nanoparticle "cocktails" can be loaded and efficiently delivered to the disease site. As demonstrated in the studies reviewed here, once the S2NPs are loaded into the S1MPs, a variety of novel properties emerge, which enable the design of new and improved imaging contrast agents and therapeutics. For example, the loading of the MRI Gd-based contrast agents onto hemispherical and discoidal S1MPs significantly increased the longitudal relaxivity (r1) to values of up to 50 times larger than those of clinically available gadolinium-based agents (~4 mM(-1) s(-1)/Gd(3+) ion). Furthermore, administration of a single dose of MSVs loaded with neutral nanoliposomes containing small interfering RNA (siRNA) targeted against the EphA2 oncoprotein enabled sustained EphA2 gene silencing for at least 21 days. As a result, the tumor burden was reduced in an orthotopic mouse model of ovarian cancer. We envision that the versatility of the MSV platform and its emerging properties will enable the creation of personalized solutions with broad clinical implications within and beyond the realm of cancer theranostics.